1. Field of the Invention
The present invention relates to a mixture comprising a sulfonate group-containing compound and to a solution composition prepared from the mixture, and more particularly, to a mixture comprising a sulfonate group-containing compound with good storage stability in organic solvents and to a solution composition prepared from the mixture.
The present invention further relates to polyurethane resin obtained using the above mixture or solution composition as a starting material, to a method of manufacturing the polyurethane resin, and to a magnetic recording medium comprising the above polyurethane resin as binder.
2. Discussion of the Background
In recent years, methods of transmitting information at high speed have developed markedly, making it possible to transmit images and data comprised of immense amounts of information. As data transmission technology has improved, there has been a demand to further increase the recording density of recording and reproduction devices and recording media for recording, reproducing, and storing information.
The use of microparticulate magnetic powder, the high-degree dispersion of microparticulate magnetic powder, and increasing the smoothness of the surface of the magnetic layer are known to be effective ways of achieving good electromagnetic characteristics in the field of high-density recording. The method of incorporating a sulfonic acid (salt) group, such as SO3Na group, into the binder is known to enhance the dispersibility of microparticulate magnetic powder. In this connection, reference can be made to Japanese Unexamined Patent Publication (KOKAI) No. 2003-132531 or English language family member U.S. Pat. No. 6,677,036, which are expressly incorporated herein by reference in their entirety.
The method of employing a sulfonic acid polyol into which a sulfonic acid (salt) group has been incorporated as a starting material diol is an example of a method of incorporating a sulfonic acid (salt) group into the polyurethane resins that are widely employed as binders in magnetic recording media. Polyester sulfonic acid polyols are among known sulfonic acid polyols. However, in polyester sulfonic acid polyols, sulfonic acid (salt) groups become localized in some of the oligomer components, and are present in a nonuniform state. Accordingly, in the polyurethanes obtained using these polyester sulfonic acid polyols as starting material polyols, the sulfonic acid (salt) groups are also present in a nonuniform state, in some cases resulting in polyurethanes containing almost no sulfonic acid (salt) groups. Such polyurethanes adsorb poorly to magnetic powder and do not afford good dispersion-enhancing effects. The polyurethane that does not adsorb may migrate to the surface of the medium, potentially generating head grime and compromising running durability.
Accordingly, it is also conceivable to employ a sulfonic acid diol monomer to obtain a polyurethane resin in which sulfonic acid (salt) groups are uniformly present. An example of such a sulfonic acid diol is N,N-bis(hydroxyalkyl)aminoethyl sulfonate described in Japanese Unexamined Patent Publication (KOKAI) Heisei No. 3-66660,
However, conventional sulfonic acid diols, including the sulfonic acid diol described in Japanese Unexamined Patent Publication (KOKAI) Heisei No. 3-66660, are highly soluble in water and poorly soluble in organic solvents. Since polyurethane resins are generally synthesized in organic solvents, it is difficult to achieve a reaction that proceeds smoothly with sulfonic acid diols that are poorly soluble in organic solvents. Good solubility in organic solvents is also desirable from the perspective of utility as a starting material in organic synthesis.
In contrast, the present inventors previously discovered a sulfonic acid (salt) group-containing polyol compound with good solubility in organic solvents, that is described in Japanese Unexamined Patent Publication (KOKAI) No. 2009-96798, published on May 7, 2009, which is expressly incorporated herein by reference in its entirety. In the present specification, the term “sulfonic acid (salt) group” includes the sulfonic acid group (—SO3H) and sulfonate groups such as —SO3Na, —SO3Li, and —SO3K. The sulfonic acid (salt) group-containing polyol described in Japanese Unexamined Patent Publication (KOKAI) No. 2009-96798 is highly soluble in organic solvents, and can thus be employed in uniform system reactions in organic solvents. Accordingly, the above sulfonic acid (salt) group-containing polyol is suitable as an organic synthesis starting material, including as a starting material of polyurethane. However, research conducted by the present inventors has revealed that when stored in an organic solvent, the above sulfonic acid (salt) group-containing polyol undergoes a substantial change in pH over time. It is required to improve the storage stability in organic solvent to increase the overall utility as an organic synthesis starting material of the above sulfonic acid (salt) group-containing polyol compound.